The objective of this study was to determine the effects of solution properties and light on the metal uptake and release in a nanosized, poorly crystalline manganese oxide (δ-MnO₂) system. The results from synthetic water matrices revealed that the aggregation state was strongly affected by ionic strength, Ca²⁺, and humic acid, and the particle aggregation subsequently changed the ability of δ-MnO₂ to adsorb and sequester heavy metal ions (Cu(II)). The extent of Cu(II) uptake onto δ-MnO₂ exhibited a negative correlation with the attachment efficiency value, which suggested that a lower sorption capacity could be achieved under aggregation-inducing conditions. Upon exposure to light, the adsorbed Cu(II) was released from the δ-MnO₂ surface via photoinduced dissolution of MnO₂. The concentration of Cu(II) desorbed was substantially higher when the humic acid was present together with Ca²⁺. The present investigation enables us to better understand the adsorption–desorption processes of heavy metals occurring at the MnO₂–solution interface in response to common environmental stimuli.